Immunostaining of progenitor and proliferation markers (e.g., Sox2, nestin and Ki67) showed their co-expression in cortex with T121, suggesting a possible dedifferentiation of mature astrocytes. Moreover, astrocyte differentiation marker S100beta was downregulated in T121 cells. To rule out the possibility of migration resulting in the observed cortical progenitor cells, focal induction of T121 in cortex using lenti-cre injection was performed. We found expression of Sox2 and nestin and reduced expression of S100beta, suggesting that the dedifferentiation was independent of the brain germinal zones. Furthermore, cortical T121 expressing astrocytes were able to generate neurospheres but astrocytes from wildtype cortex failed to do so. These cells also showed self-renewability and multilineage ability, emphasizing their dedifferentiated status upon Rb-TS inactivation. Our preliminary results show that even 200 T121 neurosphere cells have tumor forming ability. Molecular analysis is underway to decipher the mechanism behind dedifferentiation. With the Rb pathway known to be altered in more than 75% of glioblastomas, our results are a step in the direction of unraveling the process of tumor initiation for this deadly disease.